In order to achieve a next generation long-distance and large-capacity communication system, a technology to apply digital signal processing to a drive signal for an optical modulator provided in an optical transmitter has been researched.
For example, it has been researched that an optical transmitter available to perform digital signal processing on a transmission data signal used for a drive signal of an optical modulator to generate a modulated signal light with any of various optical waveforms.
Further, as an example of a technology to increase a transmission capacity, a multilevel modulation technique such as a quadrature phase shift keying (QPSK) or a quadrature amplitude modulation (QAM) has also been researched.    Patent Document 1: JP 2007-82094 A    Patent Document 2: JP 2006-270909 A
When a phase variation according to the digital signal processing is applied to the transmission data signal used for the drive signal of the optical modulator, a sign of the transmission data signal may be inverted due to a relation between a sign of the transmission data signal and an operating point of the optical modulator set by a bias control. In other words, there is a possibility that the phase variation of the modulated signal light does not correspond to an expected (or desired) phase variation according to the digital signal processing.